Conventionally, a determination technique which electrically determines the presence of a person in a seat in a vehicle using a phenomenon that the capacitance between two electrodes changes depending on the presence of the person, is known (Japanese Unexamined Patent Application, First Publication Nos. Hei 3-233391, Hei 7-270541, and 2000-249773). Moreover, a determination technique which determines an occupant's body, posture, and the like by a change in the capacitance between a plurality of conductive electrodes arranged in a central part of the steering wheel and which is usable for controlling an airbag when an impact occurs, is also known (for example, refer to Japanese Unexamined Patent Application, First Publication No. Hei 11-337427).
Incidentally, such systems have been conventionally used for airbag apparatus of vehicles where an airbag is deployed if it receives an impact regardless of whether the occupant on the passenger's seat is an adult or child. Recently, however, it has been understood that it is instead safer not to deploy an airbag in the case of a child with a small body. Therefore, an apparatus has been considered where an occupant determination apparatus is equipped in the seat on the passenger's seat side for controlling an airbag to deploy in the case of an adult but not to deploy in the case of a child when an impact occurs. However, it has been difficult to precisely control an airbag and the like according to the size of occupants in the conventional determination technique which simply determines a person's presence, posture, and the like by a change in the capacitance between a plurality of electrodes.
However, various occupant determination apparatus which enable determination of whether an adult or child, have been recently proposed and disclosed in Japanese Unexamined Patent Application, First Publication Nos. Hei 10-236269, Hei 11-258354, Hei 11-271463, Hei 11-334451, and 2002-36929, and the like.
FIG. 14 shows a schematic configuration of an example of a conventional occupant determination apparatus which enables determination of whether an adult or child. This occupant determination apparatus comprises; a plurality of antenna electrodes 1001 and 1002; a switch 1003 which switches the connections of the plurality of antenna electrodes to a transmission side and a receive side, a transmitter 1005; a current/voltage converting circuit 1004 which converts a current flowing to the electrodes into voltage, an interelectrode capacitance detecting circuit 1006 which detects an interelectrode capacitance from the view of the transmission side, which exists between the transmitter 1005 and the antenna electrodes on the transmission side, by the output of the current/voltage converting circuit 1004, and an occupant determination circuit 1007. This occupant determination apparatus determines whether an adult or a child by the change in capacitance between the antenna electrodes obtained by the interelectrode capacitance detecting circuit 1006. In this manner, in the conventional occupant determination apparatus shown in FIG. 14, utilizing the phenomenon that the capacitance between the antenna electrodes changes by the difference in the area of an occupant covering the antenna electrodes, this change of capacitance is measured for enabling determination of whether an adult or child.
However, the interelectrode capacitance detected by the occupant determination apparatus as shown in FIG. 14 includes data of the area of the occupant covering the respective electrodes (A1, A2, to AN) and the distance between the respective electrodes and the occupant (X1, X2, to XN). Therefore, for example, an adult who is sitting down on a cushion in a seat in a position far from the electrodes, and a child who is sitting down directly on a seat in a position close to the electrodes cannot be discriminated only by the magnitude of the detected value of the interelectrode capacitance as shown in FIG. 15A to FIG. 15C. That is, there is a disadvantage in that it is difficult to realize accurate determination of the size of an occupant by the determination method for determination of whether the occupant is an adult or a child, simply by the magnitude of the capacitance between the antenna electrodes.
FIG. 16 shows an example of a conventional occupant determination apparatus provided with means for solving the problem where it becomes difficult to discriminate between an adult in a position far from the electrodes and a child in a position close to the electrodes as shown in FIG. 15A to FIG. 15C. In this occupant determination apparatus, an occupant area (A) covering the electrodes is calculated by a relation between an upper layer electrode and a lower layer electrode of double layered electrodes, which are separated by a predetermined thickness d. Specifically, the arrangement is such that electrodes are provided on both side surfaces of an insulating material with a thickness of d, and the electrode on the surface close to the occupant is an upper layer electrode and the electrode on the surface far from the occupant is a lower layer electrode. In the occupant determination apparatus having such an electrode configuration, for example, an occupant area covering an electrode 1202 (A2) is calculated as follows.
Firstly, assuming that the position of a lower layer electrode 2 is in the upper layer electrode position, a virtual capacitance Cvt (2) is estimated from the capacitances of upper layer electrodes Ctp (1) and Ctp (3) on both adjacent sides as follows.
      Cvt    ⁡          (      2      )        =                    Ctp        ⁡                  (          1          )                    +              Ctp        ⁡                  (          3          )                      2  
Moreover, the capacitance Cbt (2) of the lower layer electrode 2, and the virtual capacitance Cbt (2) assuming that the position of the lower layer electrode 2 is in the upper layer electrode position, can be represented by the following equations, with the same occupant area (A2) and a different distance between the electrodes and the occupant (X).
            Cvt      ⁡              (        2        )              =          ɛ      ⁢                        A          2                          X          2                                Cbt      ⁡              (        2        )              =          ɛ      ⁢                        A          2                                      X            2                    +          d                    
If the distance X2 between the electrodes and the occupant is eliminated from the above equations, the occupant area A2 can be obtained from following equation.
      A    2    =                              Cvt          ⁡                      (            2            )                          ×                  Cbt          ⁡                      (            2            )                                      ɛ        ⁡                  (                                    Cvt              ⁡                              (                2                )                                      -                          Cbt              ⁡                              (                2                )                                              )                      ⁢    d  
As seen from the above equation, calculation of the occupant area with the influence of the distance between the occupant and the electrodes eliminated, becomes possible.
However, in the occupant determination apparatus as shown in FIG. 14, capacitance is present between an occupant and surrounding car-body metal parts (GND) other than the antenna electrodes. Moreover, a normal occupant of the seat cannot be always in the same posture and takes various postures. Therefore, the capacitance Cg between the occupant and the car-body metal parts (GND) also changes corresponding to the occupant's posture. As a result, the measured value of the capacitance between the antenna electrodes which is a composite value of the capacitances including the capacitance Cg between the occupant and the antenna electrodes, includes errors due to the capacitance component which changes with the posture of the occupant. Specifically, since the capacitance Cg changes to a large value when the occupant approaches or contacts the metal parts such as the chassis of the automobile, then in such a case, the measured value of the capacitance between the antenna electrodes changes greatly. Therefore, there is a disadvantage in that it is difficult to realize an accurate determination of the size of the occupant, by the determination method for determination of whether the occupant is an adult or a child simply from the magnitude of the capacitance between the antenna electrodes.
Moreover, in the occupant determination apparatus of the configuration shown in FIG. 16, the structure becomes complex since the electrodes which become sensors need to be a double layered structure. Furthermore, the thickness d changes due to the occupant weight, depending on the insulating material where an upper layer electrode and a lower layer electrode are formed on both side surfaces. Therefore, calculation accuracy of the occupant area is decreased. In order to suppress this accuracy decrease, it is necessary to add some means for measuring the thickness d of the insulating material when the occupant is sitting down, which results in the problem that the structure becomes more complex and cost is also increased.